If they are large enough, caverns will feel like the outdoors; they might even be plumbed for "rain" and specially vented to create "wind." Artificial weather will keep the air crisp, while artificial light sources - from vast LED arrays, fiber pipes carrying light from the surface, genetically engineered extra-phosphorescent lichen - will infuse this superspace in an eternal dawn. Sunbathers, though, will need to call for the elevator.
And, as always, artists are the shock troops of gentrification:
If history is any guide, artists will be the first to actually move underground full-time. They have a knack for converting industrial and commercial spaces into highly desirable residential real estate. Looking at their airy studios, we'll decide underground space isn't so dreary after all.
<SPECULATION> I wonder how the economics will work; will above-ground real estate become a highly-prized status symbol for the ultra-rich? Or perhaps pollution, ozone depletion or other catastrophes (nuclear fallout perhaps?) will make "up top" into a slum inhabited only by outlaws and untouchables; and as the mole people tuck their children into their beds belowground, they'll tell them blood-curdling tales about the monsters and fiends who live on the surface (much as have been told throughout the aeons about any desolate wastelands outlaws take refuge in). </SPECULATION>
or will it be something like the subterranean community depicted in <b>Demolition Man</b>? :)
Well not on that scale both Montreal and Toronto have large "underground cities" When the subways were built vast underground malls, offices, and hotels were also built. In Montreal one could visit the city and only see the sky twice. One reason why this works is that montreal is an island and has very little free space left.Anyway it all looks and feels like a giant shopping mall, as they usually only have skylights anyway.
Do people live underground in Montreal?
The other thing about Montreal that helps the underground thing is that it's cold.
Two words. Coober Pedy.
I doubt they'd want to do this in cities near to sea-level, like London or New York, especially in the light of global warming.